Stack molding provides an advantage over single molding in that it enables the output of an injection molding machine to be at least doubled without significantly increasing its size. Stack mold configurations such as shown in U.S. Pat. No. 5,846,472 to Rozema, and U.S. Pat. No. 5,229,145 to Brown generally employ a stationary first platen, a movable center platen and a movable second platen. The mold cavities are conventionally located on opposing faces of the movable center platen. The movable center platen and the second movable platen reciprocate to open and close the mold cavities during a production cycle. In a stack molding apparatus, the manifold system extends through the center platen in order to reach the mold cavities located on each side of the center platen via an equal path length.
Typically, multi-cavity stack molds use a valve gated melt transfer nozzle, which is coupled to the movable platen, for delivering melt from the extruder nozzle of the injection molding machine to the manifold. The melt transfer nozzle moves into and out of engagement with a second valve gated melt transfer nozzle that is linked to the extruder. The manifold delivers melt from the melt transfer nozzle into injection nozzles that are associated with each individual mold cavity.
As a result of the reciprocating action of the movable platens, the melt transfer nozzles are continuously coupled to and decoupled from one another. In prior art valve gated melt transfer nozzles, this results in drooling and stringing between the nozzles, which is undesirable.
There are a large number of variables in a multi-cavity injection molding process that affect the quality of the molded parts produced. One such variable is shear induced flow imbalance. As the melt flows through the manifold, the melt near the perimeter of the melt channel experiences high shear conditions due to the relative velocity of the melt flow with respect to the stationary boundary of the melt channel, whereas the melt near the center of the melt channel experiences low shear conditions. As such, the shear rate and temperature, and therefore the viscosity vary both along and across the melt channel. When the melt channel splits into two branches, the center to perimeter variation becomes a side-to-side variation after the split. This side-to-side variation typically results in a variation in melt conditions from one side to the other of the parts molded from each of the runner branches. If the melt channel branches out to deliver melt to four or more mold cavities, the melt in each of the branches will be different, which will result in variations in the product created in each of the mold cavities.
The melt that is delivered from the extruder is often not evenly balanced. Further, when valve gated melt transfer nozzles are used to transfer melt from the extruder to the manifold, the melt flow imbalance is worsened due to the effect of the valve pins partially obstructing the melt flow. Therefore, in order to reduce the magnitude of the melt flow imbalance effect at the mold cavities, it is necessary to ensure that the melt entering the manifold is evenly distributed across the melt channel cross-section.
It is therefore an object of the present invention to obviate or mitigate at least one of the above disadvantages.